I'm importing grayscale elevation images (bmp and jpeg) into a program I use for 3d CNC terrain carving. I've been downloading Hi-Res terrain corrected DEM files from ALOS PALSAR. I bring them into ER Viewer and export them as bmp files.

When I extrude the bmp the terrain is stepped, which onscreen grayscale looks smooth and does not indicate.

What am I doing wrong here. It must be something I don't understand about raster files and the way they behave. I just want to have raster files that are smooth and not stepping. I'm calling it stepping as its not exactly like contour lines but it will be noticable when I come to carve these.


This is likely a difference in bit depth. Whatever format the DEMs are provided in probably stores each pixel as a 32 or 64 bit float. These can store values out to a handful of decimal places and therefore represent a height change between neighboring pixels in a fraction of a unit. The GDAL docs suggest that the BMP format can only store 8 bits per color channel. This would limit the pixel values to whole numbers from 0-255, basically cutting off the decimal precision entirely. The difference between neighboring pixels can no longer be less than one unit (a meter?), and so you get that stair-step effect.

Check what other formats your CNC software can handle. TIFF is a common one that can handle higher-precision data types.

  • Thats very helpful. Tried tif and jpeg and both much better – carl Nov 9 '18 at 9:33

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